TRANSPIRATION OF PLANTS. 89 



whicli presumably acts as a reservoir of water), the others, having 

 fibrous roots, gave a steadily decreasing loss day by day, showing 

 no maximum or miuimum, except in some cases with clear glass on 

 the sixth day, on which the loss was greater than in the preceding. 



One suspects, therefore, that roots naturally grown in soil 

 and transferred to water, cannot carry on their function of ab- 

 sorption in a normal manner beyond a very short period of time. 

 My experience seems to corroborate that of Sachs*, quoted by 

 Duchartref : — '" Ce physiologiste a reconnu que les racines qui 

 se sont produites dans la terre ne peuvent vegeter ensuite dans 

 I'eau et que reciproquement celles qui ont pris naissance dans 

 I'eau ne peuvent reraplir leurs fonctions dans la terre." 



The plan I finally adopted was to grow small plants in miniature 

 pots, 2 inches high and nearly 2 inches in diameter. These can 

 be entirely wrapped up in gutta-percha sheeting, wiiich is carefully 

 bound round the stem of the plant with cotton-wool within and 

 around the stem. This effectually prevents any evaporation from 

 the surface of the earth or pot; and all loss of weight is due to 

 the transpiration from the exposed surface of the plant alonej. 



My experiments were made upon Lettuce, Box, Echeveria, 

 small seedling Palms, Perns, Cactus, and many other kinds of 

 shrubs and herbs ; having selected them with very various de- 

 grees of density in the epidermis, as well as of dilferent families. 

 The results would seem to entirely corroborate the conclusion of 

 Wiesner, that transpiration is mainly eiFected by the Eed, Blue, 

 and Violet rays ; while the (optically) brightest rays of yellow 

 and green are generally less able to effect it, even if they do not 

 liinder it, I emphasize this sentence, as there appear to me to be 

 grounds for coming to such a conclusion, as will be seen hereafter. 



Description of Experiments. — The experiments were all con- 

 ducted in a room with one window of JN". aspect, into which the sun 

 never entered, except just before setting in midsummer, and then 

 only at one corner of the window. The light was, moreover, par- 

 tially obscured by foliage of high trees in front of it. The window 

 was never opened, and the temperature varied but very little, the 

 maxima ranging from 61° to Qif F. by day, and the minima from 



Eot. Zeit. 1860, p. 113. 



t Elements de Botanique, 2"^^ ed. 1877, p. 289. 



X Eveu in this case we meet with a difficulty, in that after several dajs the 

 absence of air to the roots is liable to cause small and delicate plants to sufl'er \ 

 BO that other precautions must be taken with them, in not allowing the soil to 

 be too wet, and in admitting air from time to time. 



